Chemistry 10 Lab – Mole Ratios in a Reaction Producing H2 Gas

Aim/Hypothesis: The purpose of this investigation is to determine the mole ratio of H2 gas product to Mg reactant in a chemical reaction.
If the mole ratio of H2 product to Mg reactant is ___________ as shown by the balanced chemical equation, then a graph of moles of H2 vs. moles of Mg will appear to be ___________________________________________________________________

Background:
Mg reacts with HCl in the following balanced chemical equation:

A balanced chemical equation indicates

Gases from reactions can be collected over water in a graduated gas collection tube (Figure 1). The volume of a gas is dependent on the pressure. When a gas is collected in a tube immersed in a water reservoir, the pressure is dependent on the difference between the level of water in the tube and the water reservoir, as in a manometer. A gas in a manometer will be at atmospheric pressure when the liquid in the two arms of a manometer are at equal levels. This can be done with a gas collection tube by immersing the tube in a tall cylinder until the water level in the gas collection tube and tall cylinder are equal (Figure 2). The pressure of the gas in the tube will then be atmospheric pressure and the corresponding volume can be read. The combined gas law can be used to transform volume of gas under one set of conditions to volume under a second set of conditions. It is based on

Apparatus/Materials:
Gas collection tube, water, 150 mL beaker, 250 mL beaker, 6 M HCl, magnesium ribbon, tall 500 ml or 1000 ml graduated cylinder, thermometer, barometer, tray or dish for water reservoir, sandpaper, balance (0.001 g or better)

Hazards: 6 M HCl is corrosive. Wear goggles, gloves, and apron. Wash hands after you are done.

Method: Practice filling a gas collection tube with water, immersing it in a tall cylinder, and checking how the volume changes depending on the depth of the immersion. Practice until you do not lose any water in the tube during the immersion process. Sand a length of magnesium ribbon until both surfaces are shiny, removing the oxide coating. Check the volume of your gas collection tube. For 100 mL tubes, cut two different lengths of magnesium ribbon between 0.5 and 3.0 cm and mass them to the nearest 0.001 g. For 50 mL tubes, cut lengths between 0.5 and 1.5 cm. Fold the ribbons into a shape that will fit through the opening of the gas collection tube.

Fill your water reservoir. Into your gas collection tube, carefully pour in 6 M HCl so the gas collection tube is about 1/5 full. Then carefully